1. Field of the Invention
The present invention relates to an improvement in detecting deviation of scanning lines of a light beam which is employable in reproduction of images, production of printed circuit boards and the like.
More particularly, the present invention relates to detection of deviation of scanning lines in a light beam scanning apparatus including a rotary or vibratory type light deflector for deflecting a light beam modulated by an image signal to project the light beam onto a photosensitive surface for each scanning line.
The present invention is particularly effective for detecting deviation in a subscanning direction that is substantially perpendicular to a main scanning direction in which the light beam runs on the photosensitive surface along each scanning line.
2. Description of the Background Art
In light beam scanning apparatuses, the surface of a photosensitive material or the like is scanned with a light beam in a main scanning direction while relatively moving the light beam and the surface to be scanned in a subscanning direction that is substantially perpendicular to the main scanning direction, to expose the surface to be scanned. A light deflector is provided such a light beam scanning apparatus, which light deflector is operable to receive a light beam modulated by an image signal to deflect and project the light beam onto the surface to be scanned for each scanning line. The light deflector may be of a rotary or vibratory (pivotal) type, for example, a polygon mirror, a galvano mirror and a hologram disc.
In scanning of a light beam through a rotary or vibratory light deflector, the light beam sometimes scans out of a predetermined target scanning line due to stationary and unstationary deviation of deflection in the light deflector. In a polygon mirror, for example, each deflecting surface of the polygon mirror often has a portion which is not parallel to the rotation axis of the polygon mirror due to errors in shaping respective deflecting surfaces through a cutting process. The reflection of the light beam from this portion causes the scanning position of the light beam to stationarily deviate from the target scanning position on the surface to be scanned. Further, when the polygon mirror is rotated about the rotational axis, the respective angles of the deflecting surfaces to the designed rotational axis unstationarily deviate due to the deviation of the rotational axis, so that scanning lines unstationarily deviate in the subscanning direction.
In general, deviation of deflecting surfaces from designed planes are called as "inclination of deflecting surfaces". The stationary deviation due to the shaping errors of the deflecting surfaces is generally called as a "static inclination of deflecting surfaces", and the unstationary deviation of the deflecting surfaces due to the deviation of the rotational axis is generally called as "dynamic inclination of deflecting surfaces".
The positional deviations of scanning lines cause inferior quality of recorded images. It is hence necessary to correct the scanning position of the light beam to coincide with the predetermined target position. In a conventional light beam scanning apparatus adapted to make such correction, the total deviation of the practical scanning positions of the light beam from the target scanning positions in the subscanning direction is calculated through an additional optical system and the light beam is deflected in the subscanning direction to scan the target positions as a function of the total deviation.
Referring to FIG. 11 depicting such a conventional apparatus, a monitor light beam 102 is projected from a monitor light source 100 onto a light-reflecting top surface 101a of a main light deflector 101 rotating on a rotational axis 101c. A position detector 104 detects the deviation of a reflected light 103 of the monitor light beam 102. The detected value represents unstationary deviation b of a deflecting surface 101b of the light deflector 101 in rotation.
Light-position detectors 106 and 107, which are disposed respectively at scanning start and end points of a scanning line 105, detect light beam positions c and d at the scanning start and end points in a subscanning direction. The unstationary deviation b is subtracted from the light beam positions c and d detected by the light-position detectors 106 and 107, to obtain stationary deviation e and f at the scanning start and end points in the subscanning direction. A computing unit 108 calculates stationary deviation g at each position in each main scanning as a function of the stationary deviation e and f through linear interpolation. The total deviation is obtained by adding the unstationary deviation b to the stationary deviation g calculated in the computing unit 108. A sub-deflector 110, which is an AOD in FIG. 11, is provided between optical systems 111 and 112. The sub-deflector 110 receives a signal representing the total deviation g+b through a drive circuit 113. A recording light source 109 generates a recording light beam. The recording light beam is deflected by the sub-deflector 110 in response to the signal to cancel the total deviation g+b.
In the conventional light beam scanning apparatus as above described, it is necessary for the main light deflector 101 to be formed with the light-reflecting top surface 101a for reflecting the monitor light beam 102 in addition to the deflecting surfaces 101b, in order to detect the unstationary deviation which is necessary to obtain the total deviation. However, high accuracy is required for shaping the reflecting top surface 101a, resulting in increasing costs of the main light deflector 101.
Furthermore, in order to calculate the total deviation, the conventional apparatus needs the additional optical components 100, 104 and others. This causes the problem that costs of the apparatus rises.